This invention relates to flexible viscoelastic polyurethane foams used in bedding and furniture cushions. Produced at above atmospheric conditions from certain foaming mixtures, the foams of this invention provide improved retention of viscoelastic characteristics, improved hand touch and are more readily produced with conventional foaming equipment.
Cellular polyurethane structures typically are prepared by generating a gas during polymerization of a liquid reaction mixture generally comprised of a polyester or polyether polyol, an isocyanate, a surfactant, catalyst and one or more blowing agents. The gas causes foaming of the reaction mixture to form the cellular structure. The surfactant stabilizes the structure.
Once the foam-forming ingredients are mixed together, it is known that the foam may be formed under either elevated or reduced controlled pressure conditions. PCT Published Patent Application WO 93/09934 discloses methods for continuously producing slabs of urethane polymers under controlled pressure conditions. The foam-forming mixture of polyol, isocyanate, blowing agent and other additives is introduced continuously onto a moving conveyor in an enclosure with two sub-chambers. The foaming takes place at controlled pressure. Reaction gases are exhausted from the enclosure as necessary to maintain the desired operating pressure. The two sub-chambers, a saw, and airtight doors are operated in a manner that allows for continuous production of slabstock polyurethane foam.
Polyurethane foams with varying density and hardness may be formed. Hardness is typically measured as IFD (xe2x80x9cindentation force deflectionxe2x80x9d). Specifically, IFD25 is the force required to compress the foam to 25% of its original thickness or height using the test method set out in ASTM D-3574. Tensile strength, tear strength, compression set, air permeability, fatigue resistance, support factor, and energy absorbing characteristics may also be varied, as can many other properties. Specific foam characteristics depend upon the selection of the starting materials, the foaming process and conditions, and sometimes on the subsequent processing. Among other things, polyurethane foams are widely used for bedding and furniture cushioning applications.
Viscoelastic polyurethane foams are characterized by high vibration damping, body conformance and slow recovery from compression. Viscoelastic foams have gained popularity for bedding applications because such foams are advertised as reducing pressure points, which are believed to cause tossing and turning during sleep.
All or almost all polyurethane foams undergo a transition from a rigid glass-like state to a soft rubber-like state. Over that transition, the foam is viscoelastic. For a typical slabstock polyurethane foam, the viscoclastic transition occurs at about xe2x88x9250xc2x0 C., which is termed its glass transition temperature. Such a low glass transition temperature limits the usefulness of such foams for room temperature applications.
To obtain viscoclastic behavior in a polyurethane foam intended for room temperature applications, one possible approach is to shift the glass transition temperature nearer to room temperature by using a lower molecular weight polyol in combination with a lower isocyanate index. However, the low isocyanate index can result in a foam with poor fatigue resistance. To compensate for poor fatigue resistance, the industry trend has been to raise the density of the resulting foam. Yet increased density can cause significant processing difficulties where achieving increased density in conventional foaming processes usually requires lowering the water content, which leads to less urea formation and subsequently low foam permeability, and even shrinkage. Very often, to prevent shrinkage in higher density foams, cell openers are added to the foam-forming mixtures. But the resulting foams formed with a cell opener can have a coarse cell structure and a rough outer surface. Such a coarse structure and rough surface conflicts with consumer expectations for a generally fine cell structure with a smoother surface that is perceived to offer better comfort.
The polyurethane foaming reaction is exothermic. Another significant problem resulting from foaming with lower water content is that the reaction exotherm generally is reduced. Foam mixtures with higher water content require more isocyanate, and thus generate a greater amount of heat to promote the foaming reaction to completion. With a lower exotherm, the foam cure is slower, and may not be sufficiently complete at the end of the conveyor in a conventional slabstock foaming production equipment. The poor cure and the relatively low foam bun height profile make it difficult for the crane or handling equipment to pick up the foam bun from the end of the pourline conveyor to move the bun to a suitable location to complete the cure. If the foam bun is lifted prematurely, it can be damaged. In addition, in most cases, the bottom and sides of the foam conveyor in the pouring equipment are lined with plastic sheets to keep the rising foam mixture from sticking to the conveyor surface as it is conveyed away from the mix head or trough that introduces the foaming mixture onto the conveyor. A poor cure as experienced with foams produced at lower exotherms can result in a weak bond forming between the foam bun and the plastic sheets. When this occurs, the plastic sheet more easily delaminates from the foam, which can jam up the rollers in the conveyor and lead to a costly shutdown of the foam manufacturing process. To prevent these problems, the viscoelastic polyurethane foam bun frequently must be left on the pourline for a longer time, as long as 3 hours compared to about 5 minutes in conventional slabstock foam grades. These processing difficulties have made the viscoelastic foam very production-unfriendly.
Commercially available viscoelastic foams also have exhibited variable performance. Unfortunately, there is no ASTM or other standardized test for measuring foam viscoelasticity. One common way to quantify viscoelasticity is to measure the visco recovery time. In that measurement, a predetermined load is applied to the foam for a fixed amount of time, typically resulting in a significant indentation. After the load is removed, the time it takes the foam to recover to its original height or to a predetermined height is measured. A longer recovery time indicates a higher degree of viscoelasticity. The load size and shape and the foam shape geometry in such tests have not been standardized. The viscoelasticity measurement is further complicated because the viscoelasticity property does not remain constant, but tends to deteriorate over time in low-index foams. In general, the lower density products have a lower initial viscoelasticity and poorer retention of viscoelasticity over time.
High density viscoelastic foams with improved retention of the viscoelastic characteristics and improved hand touch that can be produced efficiently and economically are continually sought for bedding and furniture applications. The prior art does not disclose production-friendly methods for making high density, fine cell viscoelastic polyurethane foams.
According to the invention, flexible, fine cell, high density viscoelastic polyurethane foams with long recovery time and excellent retention of viscoelasticity are produced using a method comprising preparing a foam reaction mixture and foaming that mixture at above atmospheric pressure conditions, preferably at pressures in the range of 1.05 to 1.5 bar (absolute), most preferably 1.1 to 1.3 bar (absolute). The reaction mixture contains (a) a polyol mixture of (i) about 50 to 95 percent by weight total polyols of a polyether polyol having from 0 to 40 percent ethylene oxide groups, and having a hydroxyl number in the range of about 120 to 220 and a functionality from 2.7 to 3.3, and (ii) about 5 to 50 percent by weight total polyols of a graft polyol having a ratio of styrene to acrylonitrile of about 70 to 30, and having a hydroxyl number in the range of about 25 to 50 and a functionality from 2.5 to 3.0; (b) an organic polyisocyanate selected from the group consisting of methylene diisocyanate and methylene diisocyanate mixed with toluene diisocyanate, wherein if a mixture of methylene diisocyanate and toluene diisocyanate is used, the polyisocyanate mixture comprises from about 5 to 20 percent by weight toluene diisocyanate and about 80 to 95 percent by weight methylene diisocyanate, wherein at least 50 percent of the methylene diisocyanate is 4,4xe2x80x2 methylene diisocyanate, and wherein the isocyanate index is in the range of 70 to 95; and (c) from about 1.2 to 2.5 parts per hundred parts polyol of water as a blowing agent.
Most preferably, the foam-forming composition contains up to 2 parts per hundred parts polyol of an amine catalyst, up to 2 parts per hundred parts polyol of a surfactant, up to 0.5 parts per hundred parts polyol of an organotin catalyst, and up to 0.3 to 2 parts per hundred parts polyol of a cross linking agent.
The resulting high density polyurethane foams have densities in the range of about 4 to 7 pounds per cubic foot, preferably about 4 to 5 pounds per cubic foot, a low surface roughness (preferably Ra under 0.20 mm as measured by perthometer), a high degree of initial viscoelasticity and excellent retention of viscoelasticity over time. In addition, such foams preferably have a finer cell size of about 70 pores per linear inch or finer.